This is a project to reverse-engineer and fix the BBC Micro game 'Bird Strike' by Andrew Frigaard
The original had a fault that after level 13 graphical corruptions appear and the game crashes when level 23 loads.
This source code was obtained by disassembling the original binary file "BIRDSK2" with WFDis and Beebdis.exe (Beebdis control file included).
It can then be assembled with beebasm.exe (eg beebasm.exe -i birdsk2.asm) to produce a 6502 binary.
To run, load the binary into the memory of a BBC Micro with
PAGE=&3000
*LOAD birdsk2 1200
CALL &1300
To play the original game either load BIRDSK1 in between changing page and `*LOADing above, or do the following:
PAGE=&3000
*LOAD birdsk2 1200
?&3527 = &A4
?&3528 = &E0
?&3529 = &A6
?&352A = &FF
CALL &1300
This emulates the loader program BIRDSK1 being loaded, which the original BIRDSK2 copies some memory locations from - possibly as an anti-tamper/copy protection method.
A number of BeebEm save states have been added for anyone wanting to see the glitch without having to play through the levels, or for debugging/analysis purposes, etc.
They are:
- Fresh game loaded, never been played. Press space and enjoy!
- Level 9 - a few levels before the first glitch I can spot.
- Level 12 - the level before the first visible glitch
- Level 13 - the first visible glitch - a 'stuck' enemy bomb at the top of the screen
- Level 20 - many glitches present by this point - cloud/pigeon sprite corruption etc
- Level 22 - the last level before the game crashes completely. Down the plane and it's all over!
There is also a B-em save state for Level 22.
The save states have invincibility enabled. To disable enter the following command in the BeebEm debugger:
c 2238 A9
(To re-enable repeat but replace A9 with 60 (RTS))
May '21- Labels from the author's original sources added as comments to birdsk2.asm. Potential 'better' fix identified - simply move the L1A09 (tm+1 in the original) after the two INCs.
15/02/21 Final(?) fix method implemented. Should play identically to the original now, save for the glitches and crash :)
11/01/21 Improved fix implemented, which doesn't break the double-fire feature. Binary and disk image updated with this new build.
09/01/21 Added playable binary with BeebEm-format .INF file and a .SSD file containing the repaired game. These are as close to the original as possible*, so require BirdSk1 to be loaded first. Shift-Break the disk image, or replace file BirdSk2 on your Bird Strike floppy/image with the one here.
*With the exception of giving myself a little credit :D
08/01/21 Cause of the original glitch/crash discovered! See code at label L1F03.
06/01/21 Additional comments added. Vector table data hard-coded so BIRDSK1 doesn't need to be loaded first. NOPs added so memory locations are consistent with the original binary.